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Abstract Group members do not always act cohesively when facing extra-group rivals. When benefits such as group-defence are not monopolizable, it poses an economics problem: who should contribute to public goods and who should freeload? A collective action framework compliments existing theoretical explanations for cooperation, and provides testable hypotheses about group-level behaviour based on individual costs and benefits. Using this approach, we review research on intergroup encounters in non-human primates published over the last 20 years, focusing on participation by different classes of individuals. While food- and mate-defence explain much between-sex variation in participation, rank and reproductive access frequently explain within-sex variation. In some species, individuals may use intergroup interactions to survey potential transfer locations and mating options, which might coincidently intimidate rivals. Experimental evidence suggests that when intergroup dominance is based on relative number of fighters, individual participation still varies with sex, rank, companion behaviour and dependent offspring presence. Relatively few studies have examined how factors such as relationships within and between groups or individual temperament mediate aggression. Long-term studies of multiple habituated groups and methodological advances (e.g., playback experiments) will continue to improve our understanding of how complex group-level patterns are predictable when viewed from an individual perspective.
Beehner et al. (Mon,) studied this question.